This invention relates to a nut and screw assembly and an improved nut, and more particularly to an improved precision nut and screw assembly and nut structure for use as a substitute for precision ball screw assemblies wherein the nut and screw assembly has a substantially low backlash.
Most machine tools include a table which is moveable in an x-y direction or in three coordinates, that is, an x-yz direction. The table is normally mounted on a saddle which includes a lead screw or a ball screw device to effect movement of the table in one coordinate direction. Normally, the lead screw is rotated to effect translational movement of the nut assembly which is fixed in a housing such that movement of the nut moves the housing, the latter attached to effect controlled movement of the table.
In precision machine tools, and other precision devices, the usual choice of a translating assembly is a precision ball screw assembly.
Most frequently the choice of a precision ball screw assembly is predicated on the desire for low backlash, a significant consideration, especially with numerically controlled machine tools which must be positioned automatically and accurately under the control of electronic control assemblies.
The ball screw assembly is recognized as a highly accurate translating device, and normally includes a screw and a nut, with balls in the form of ball bearings being located between the nut and screw. In the relative movement of the nut with respect to the screw, the ball bearings roll in grooves. Thus, these devices are inherently low friction devices in which the dynamic friction of the system is essentially the same as the static friction of the system. While a ball screw is a highly accurate device, it does have some disadvantages in the sense that it is a relatively expensive assembly, for example, it is not uncommon to find ball screw assemblies priced in the range of several thousand dollars each. Part of the reason for the expense in such assemblies is the fact that the manufacture thereof requires matching of the ball elements to the nut and the screw.
While most ball screw assemblies of the precision type are capable of accuracy to the order of one-tenth of one-thousandth of an inch, the overall accuracy of the machine tool may be somewhat less because of the errors in the thrust bearings and the gear train. Accordingly, a precision nut and screw assembly which is capable of accuracy to the extent of one-thousandth or one-half of one-thousandth of an inch finds utility in roughly seventy to eighty percent of the installations which are currently utilizing the more expensive precision ball screw assembliles.
Since, as mentioned above, a ball screw involves matching of the balls, the grooves of the nut and the grooves of the screw, it is not possible to replace a worn nut without going through the expensive procedure of rematching the nut, the balls, and the screw. Accordingly, the usual procedure is to replace the entire ball screw assembly, an expensive operation. Morerover, there is not much adjustment available in the friction of a precision ball screw assembly.
There have been instances in the operation of machine tools in which metallic chips have become lodged between the ball and the screw or between the ball and the nut of a ball screw assembly resulting in a catastrophic failure of the ball element as it attempts to roll over the metal chip. This failure is in the nature of a compressive failure sometimes referred to as "Brinnelling" which requires a complete replacement of the precision ball screw assembly. Moreover, if for some reason the nut is advanced off the end of the screw such that the balls fall out of their position, they cannot be replaced without precision matching and placement in the proper position within the ball screw assembly.
In the case of larger machine tools in which a relatively heavy table is moved in a vertical position under the control of a precision ball screw, it is usually necessary to lock the table in the vertical position by some physical locking device, or in the alternative, to maintain the power on the screw so as to hold the table in the vertical position. The reason for this is that a ball screw has a relatively low coefficient of friction and the weight of the table and the weight of the nut tend to cause the entire assembly to coast to its lowermost position. Additionally, because of the low coefficeint of friction, which for precision ball screw assemblies is very very low, there is a tendency for the work table to "float" even in horizontal x-y movements.
The operation of a ball screw normally involves contact between the rollers of the ball screw and one side wall of the groove on the screw in moving in one direction, and rolling contact between the balls and the other side wall in moving in the opposite direction. Thus, in some assemblies of ball screws, there are two sets of balls which alternately engage one side wall of the screw dependding upon the direction of movement. As the balls wear, there is corresponding wear on the screw, and because of the nature of ball screws, it is not possible to adjust for the wear which takes place. In view of the fact that most machining operations are normally carried out in what is best described as the mid portion of the machine travel, the major wear on the screw occurs in that portion of the screw generally around the mid portion of the travel or several inches or feet in each direction, depending upon the size of the machine tool. Thus, the accuracy of the equipment in that portion of its movement in which most of the machining is done gradually decreases. If it is attempted to adjust the ball screw to compensate for the slack due to wear in the center portion of the travel where there has been relatively less wear. The result is that the translating motion of the table in any of the coordinate directions is nonuniform, in the context of higher frictional movement at the extreme ends of the range and considerably less friction in the mid range, depending upon the extent of wear.
Nonetheless, it is quite apparent that ball screws of the precision type generally perform well and have received wide acceptance because of the precision and relatively low friction which is characteristic of a precision ball screw.
Lead screw and nut assemblies are known in the prior art in which the nut is fabricated of a plastic material, see for example, U.S. Pat. No. 3,698.257 issued Oct. 17, 1972, and assigned to the same assignee.
U.S. Pat. No 3,190,132 of June 22, 1965, describes a tuner mechanism which includes a floating sleeve of plastic in which the threads of the plastic differs from the threads of the worm so as to prevent backlash between the worm shaft and the plastic sleeve.
U.S. Pat. No. 3,081,664 of Mar. 19, 1963, describes a close tolerance antifriction assembly in which a film coating of a plastic material is adhered to the mating surfaces of one of the threaded members.
It will be apparent, that there are definite advantages in providing a precision nut and screw assembly having a very low backlash, for example between one-thousandth of an inch and one-half of one-thousandth of an inch and in which the cost for such an assembly is substantially less than that for a comparable assembly of the ball screw type. With such a precision nut and screw assembly, there are added advantages if it is possible to replace the nut should the same become worn thereby reducing substantially the cost of maintenance and replacement, a definite factor in the use of ball screw assemblies.
In some installations, the use of a nut and screw assembly which provides substantially reduced backlash and precision, but which does not "coast" as does a ball screw assembly, is definitely advantageous, especially with respect to vertical translating mechanisms for use with machine tools having relatively heavy work tables which must be moved in a vertical direction. That is, if the nut and screw assembly provides acceptable accuracy in its translating motion, without substantial backlash, and has sufficient friction to maintain a proper vertical position without the application of a holding torque through the motor and screw, or without the necessity for positively locking the table in the vertical position, such an improved nut and screw assembly would have definite advantages.
It would also be advantageous to provide a precision nut and screw assembly in which the screw does not undergo any considerable wear and in which the rear takes place on the nut which can be easily replaced and wherein the adjustment thereof is relatively simple to provide the proper frictional and backlash characteristics as may be desired.